A brief description will be made of a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) communication system as an exemplary mobile communication system to which the present invention is applicable.
FIG. 1 is a diagram illustrating a network configuration of Evolved-Universal Mobile Telecommunications System (E-UMTS) as an example of a mobile communication system.
E-UMTS has evolved from Universal Mobile Telecommunication System (UMTS) and the 3GPP is working on basic standardization of E-UMTS. E-UMTS is also called LTE.
The E-UMTS network may be largely divided into a UMTS Terrestrial Radio Access Network (E-UTRAN) 101 and a Core Network (CN) 102. The E-UTRAN 101 includes a UE 103, an Evolved Node B (eNode B or eNB) 104, and an Access Gateway (AG) which is located at an end of the network and connected to an external network. The AG 105 may be divided into a part for handling user traffic and a part for handling control traffic. An AG for handling new user traffic may communicate with another AG for handling control traffic via a new interface.
An eNode B manages at least one cell. An interface for transmitting user traffic or control traffic may be used between eNode Bs. The CN 102 may include the AG 105 and a node for performing user registration for the UE 103. An interface may also be used to distinguish the E-UTRAN 101 from the CN 102.
A radio interface protocol stack between a UE and a network may include a first layer (L1), a second layer (L2) and a third layer (L3) based on the three lower layers of a well-known Open System Interconnection (OSI) reference model for communication systems. Among the layers, the PHYsical (PHY) layer or L1 provides an information transfer service on physical channels. The Radio Resource Control (RRC) layer in L3 manages radio resources between the UE and the network. For the radio resource management, the RRC layer exchanges RRC messages between the UE and the network. The RRC layer may be distributed to network nodes including the eNode B 104 and the AG 105, or located in either the eNode B 104 or the AG 105.
Following is a brief description of a multi-input multi-output technique.
Conventionally, a single Transmit (Tx) antenna and a single Receive (Rx) antenna are used. Multi-Input Multi-Output (MIMO) uses a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. That is, the use of multiple antennas at both a transmitter and a receiver may increase capacity and performance in a wireless communication system. Hereinbelow, MIMO may be referred to as ‘multi-antenna’.
The multi-antenna technology does not depend on a single antenna path to receive a whole message. Rather, it completes the data by combining data fragments received through a plurality of antennas. With the multi-antenna technology, data rate may be increased within a cell area of a certain size, or system coverage may be extended with a predetermined data rate ensured. Furthermore, this technology may find its use in a wide range including mobile terminals, relays, etc. The multi-antenna technology may overcome transmission capacity problems encountered with the conventional single-antenna technology.
FIG. 2 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 2, a transmitter has NT Tx antennas and a receiver has NR Rx antennas. The use of a plurality of antennas at both the transmitter and the receiver increases a theoretical transmission capacity, compared to the use of a plurality of antennas at only one of the transmitter and the receiver. The channel transmission capacity increases in proportion to the number of antennas. Given a maximum transmission rate Ro in case of a single antenna, the transmission rate may be increased, in theory, to the product of Ro and Ri in case of multiple antennas. Ri is a transmission rate increase rate.
For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
There are two types of MIMO schemes: spatial diversity and spatial multiplexing. Spatial diversity increases transmission reliability using symbols that have passed in multiple channel paths, whereas spatial multiplexing increases transmission rate by transmitting a plurality of data symbols simultaneously through a plurality of Tx antennas. The advantages of these two schemes may be taken by using them in an appropriate combination.
To describe a communication scheme in a MIMO system in detail, the following mathematical model may be used. On the assumption of NT Tx antennas and NR Rx antennas as illustrated in FIG. 2, the maximum rank Ri of a channel matrix is given as [Math FIG. 1].
MathFigure 1Ri=min(NT,NR)  [Math.1]
Regarding a transmission signal, if NT Tx antennas are used, up to NT pieces of information can be transmitted, as expressed as the following vector.
MathFigure 2S=[S1,S2, . . . SNT]T  [Math.2]
A different transmit power may be applied to each piece of transmission information S1, S2, . . . , SNT.
Let the transmit power levels of the transmission information be denoted by P1, P2, PNT, respectively. Then the power-controlled transmission information Ŝ may be given as [Math FIG. 3].
MathFigure 3Ŝ=[Ŝ1,Ŝ2, . . . ,ŜNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Math.3]ŝ may be expressed as a diagonal matrix P of transmit power.
MathFigure 4
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Math          .                                          ⁢          4                ]            
Meanwhile, actual NT transmitted signalsx1,x2, . . . ,xNT may be configured by applying a weight matrix W to the power-controlled information vector ŝ.
The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. These transmitted signalsx1,x2, . . . ,xNT 
are represented as a vector x, which may be determined as [Math FIG. 5] below.
MathFigure 5
                                                        x              =                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                                                [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                w                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              w                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                                                                                        =                                                W                  ⁢                                      s                    ^                                                  =                WPs                                                                        [                  Math          .                                          ⁢          5                ]            
The signal vector x is represented as follows. Herein, wij denotes a weight for a jth piece of information Ŝj 
transmitted through an ith Tx antenna and the weights are expressed as the matrix W. W is referred to as a weight matrix or a precoding matrix.
The afore-mentioned transmitted signal x may be considered in two cases: spatial diversity and spatial multiplexing.
In spatial multiplexing, different signals are multiplexed prior to transmission. Accordingly, the elements of the information vector s have different values. In contrast, the same signal is transmitted in a plurality of channel paths in spatial diversity. As a result, the elements of the information vector s have the same value.
Spatial multiplexing and spatial diversity may be used in combination. For example, the same signal may be transmitted through some Tx antennas in spatial diversity, while different signals may be transmitted through the other Tx antennas in spatial multiplexing.
For NR Rx antennas, signals received at the Rx antennas,y1,y2, . . . ,yNR 
may be represented as the following vector.
MathFigure 6y=[y1,y2, . . . ,yNR]T  [Math.6]
In the mean time, when channels are modeled in the MIMO communication system, they may be distinguished according to the indexes of Tx and Rx antennas and the channel between a jth Tx antenna and an ith Rx antennas may be represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij.
The channels may be represented as vectors and a matrix by grouping them. The vector representation of channels may be carried out in the following manner.
FIG. 3 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 3, the channels from the NT Tx antennas to the ith Rx antenna may be expressed as [Math FIG. 7].
MathFigure 7hiT=[hi1,hi2, . . . ,hiNT]  [Math.7]
Also, channels from NT Tx antennas to NR Rx antennas may be expressed as the following matrix.
MathFigure 8
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Math          .                                          ⁢          8                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGNn1,n2, . . . ,nNR 
added to the NR Rx antennas is given as the following vector.
MathFigure 9n=[n1,n2, . . . ,nNR]T  [Math.9]
From the above modeled Math Figures, the received signal is
MathFigure 10
                                                        y              =                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R           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                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                              Hx                +                n                                                                        [                  Math          .                                          ⁢          10                ]            
The above-described MIMO operation is for a single user in the MIMO communication system. When the MIMO communication system operates for multiple users, multi-user diversity may be achieved. Now a description will be made of the multi-user diversity.
A description will be made below of codewords used in the MIMO communication system. A transmitter encodes transmission information using a forward error correction code prior to transmission in order to enable a receiver to correct channel errors in a typical communication system. After demodulating the received signal, the receiver recovers the transmission information by decoding the demodulated signal using the error correction code. In this manner, channel errors of the received signal are corrected during the decoding.
Aside from the error correction, a special coding process is required for error detection. In general, a Cyclic Redundancy Check (CRC) is used as an error detection code.
CRC is one of coding methods for error detection, not for error correction. Typically, the transmitter encodes transmission information with a CRC and then encodes the CRC-coded information with an error correction code. The resulting one coded unit is called “codeword”.
The numbers of rows and columns in the channel matrix H representing channel statuses are determined according to the numbers of Tx and Rx antennas. The number of rows is identical to that of the Rx antennas, NR and the number of columns is identical to that of the Tx antennas, NT. Thus, the channel matrix H is of size NR*NT.
In general, the rank of a matrix is defined as the minimum of the numbers of independent rows or columns. Accordingly, the rank of the matrix is not larger than the number of rows or columns. For example, the rank of the matrix H, rank(H) is limited as follows.
MathFigure 11rank(H)≦min(NT,NR)  [Math.11]
If the matrix is eigen value-decomposed, its rank may be defined as the number of non-zero eigen values. Similarly, in case of Singular Value Decomposition (SVD), the rank may be defined as the number of non-zero singular values. Therefore, the rank of a channel matrix physically means the maximum number of different pieces of information that can be transmitted on given channels.
A different piece of information transmitted in MIMO is referred to as ‘transmission stream’ or shortly ‘stream’. The ‘stream’ may be called ‘layer’. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of transmittable different pieces of information.
The channel matrix H is determined by
MathFigure 12# of streams≦rank(H)≦min(NT,NR)  [Math.12]“# streams” denotes the number of streams. One thing to be noted herein is that one stream may be transmitted through one or more antennas.
One or more streams may be mapped to a plurality of antennas in various manners. The mapping can be considered as follows according to MIMO types. It can be said that transmission of one stream through a plurality of antennas corresponds to spatial diversity and transmission of a plurality of streams through a plurality of antennas corresponds to spatial multiplexing. Obviously, spatial diversity and spatial multiplexing may be used in a hybrid manner.
A description will be made below of channel coding scheme and HARQ technique.
For reliable transmission, forward error correction (FEC) code is employed. Then, in receiver side, after demodulation, the information is recovered with decoding procedure.
There are many types of channel coding: for example, convolutional code and block code. In this invention, we explain turbo code, so called parallel concatenated convolutional code. A turbo code consists of two recursive systematic convolutional codes connected with a interleaver. Output coded bits consist of systematic and parity bits.
In real communication systems, large size of data block is usually segmented into multiple coding blocks for limitation of implementation. Then, actual channel encoding is done in unit of code block. After encoding, coded bits go through a channel interleaver to combat burst errors.
Finally, to match real transmission resource, a rate matching procedure is done. The rate matching may be separately done for systematic and parity bits. Circular buffer rate matching operates as follows: for a given coding rate, a part of data bits in the circular buffer is transmitted in circular manner.
In case of retransmission, if non overlapped part of data bits is transmitted, we can get coding gain, so called incremental redundancy (IR) gain.
Hybrid automatic repeat request (HARQ) is combination of channel coding and automatic request (ARQ) to improve system throughput. If a receiver decodes a data block successfully, then it sends a acknowledgement (ACK) to the transmitter. Otherwise, it sends negative acknowledgement (NACK) to the transmitter. If the transmitter receives NACK, then the transmitter retransmits the data block. If the transmitter receives ACK, then the transmitter transmits new data if it has data to send.
There are two types of HARQ operation according to retransmission timing. One is asynchronous HARQ and the other is synchronous HARQ. In asynchronous HARQ, retransmission timing is not fixed, which requires indication of whether current transmission is retransmission or not.
On the other hand, synchronous HARQ, retransmission timing is fixed after the initial transmission. For example, if the initial transmission fails, then the retransmission is always occurred at 8 transmission instants after the initial transmission. For another classification, there are also two types of HARQ operation according to redundancy versions. One is chase combining (CC) and the other is IR. In CC type of HARQ, the same data is transmitted at every retransmission, which gives SNR gain. On the other hand, IR type of HARQ, a different redundancy version may be transmitted in retransmission, which gives coding gain.
If we apply HARQ to a system with circular buffer rate matching, IR can be implemented by indicating the starting position of a retransmitted data block. The starting position in the circular buffer for transmission may be defined for each redundancy version (RV).
The aforementioned data block can be processed as described below.
Firstly, CRC is attached to a data block, so called transport block (TB). Multiple ACK/NACK may be sent if multiple TBs are transmitted in a TTI. On the other hand, single ACK/NACK may be sent even if multiple TBs are transmitted in a TTI.
For MIMO system, multiple TB can be transmitted in a transmission time instant, so called TTI. Then, each TB is segmented into multiple code blocks if the size of data block exceeds a threshold value. Each code block is encoded and rate-matched. Then, after concatenation of code block, it goes through a channel interleaver.
After channel interleaving, the data should be mapped to time, frequency, and spatial resource elements. The following is an example of mapping to spatial resource, so called layer.
Table 1 below shows an example of mapping to layer assuming 4 transmit antennas.
TABLE 1TransmissionrankMapping to layer1s1(i) = d1(i)2s1(i) = d1(i), s2(i) = d2(i)2s1(i) = d1(2i)s2(i) = d1(2i + 1)3s1(i) = d1(i), s2(i) = d2(2i)s3(i) = d2(2i + 1)4s1(i) = d1(2i)s2(i) = d1(2i + 1)s3(i) = d2(2i)s4(i) =d2(2i + 1)
In Table 1, sk(i) (k=1, 2, 3, 4) is the data mapped to the k-th layer at the i-th index and dj(i) (j=1, 2) is the data from the j-th TB at the i-th index. In rank 1, single TB is supported and it is mapped to layer 1. In rank 2, 2 TBs are supported and they are mapped to layer 1 and layer 2, respectively. In addition to that, only TB 1 is mapped to layer 1 and layer 2, which may support in retransmission only. In rank 3, 2 TBs are supported, and TB 1 is mapped to layer 1 and TB 2 is mapped to layer 2 and layer 3. In rank 4, 2 TBs are supported, and TB 1 is mapped to layer 1 and layer 2 and TB 2 is mapped to layer 3 and layer 4.
Active studies are underway in many respects regarding the MIMO technology, inclusive of studies of information theory related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring radio channels and deriving a model for a MIMO system, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc. Especially, it is necessary to conduct a study of a method for efficiently implementing uplink HARQ retransmission scheme in a MIMO system.